1. Field of the Invention
The invention involves a process and apparatus for the electroplating of metals. In particular, it involves electroplating utilizing a consumable anode.
2. Description of the Prior Art
Electroplating of metals such as, for example, copper and nickel is extensively used in the manufacture of various articles and devices. Well known is the plating of metals on iron and other metals to prevent corrosion and to increase conductivity. Recent developments include electroplating of metals in the manufacture of electronic circuits and devices. In these latter applications a considerable degree of control is required in order to obtain uniform and reproducible results.
A particular problem associated with electroplating is corrosion of various electrical connections used in the process. Particularly troublesome is the variation in current densities caused by corrosion in electrical contacts.
A particular case in point involves electroplating processes in which the plating metal is replenished using a consumable anode. Here, part of the anode assembly (called the consumable part of the anode) is made of the metal being plated and is dissolved by electrolytic action during the plating process. In order to facilitate resupply of this metal, the consumable part of the anode is usually made replaceable. Electrical contact between the consumable part of the anode and the anode rod is made through a hanging device called here an anode hook. The anode rod is connected to the electrical power supply. Conventionally, anode hooks are made from round stock in the form of hooks or eyes and are threaded into the consumable part of the anode. These hooks are hung on the anode rod. Usually, there is a multiplicity of anode hooks (with attached plating metal) hung on each anode rod and sometimes a number of anode rods in each anode assembly. In commercial manufacturing procedures where large plating tanks are run on a semi-continuous basis, a rapid and convenient procedure for replenishing the consumable part of the anode is highly desirable. In copper plating, the anode hook is sometimes made of titanium, but the anode rod is usually made of copper. Other materials either were not believed to have sufficiently high electrical conductivity, or presented the danger of contaminating the electroplating bath. Although titanium anode hooks and copper anode rods perform reasonably well, considerable care must be taken to insure that corrosion does not lead to alteration of the electrical resistance in the contacts which would produce nonuniform current distributions. This leads to considerable inconvenience in replenishing the consumable part of the anode. The consumable part of the anode is often made in the form of metal bars with the anode hooks threaded into the bars. As the metal dissolves, the threads which hold the bars on the hooks dissolves. This increases the electrical resistance between hook and metal bars and leads to nonuniform plating. In severe cases, the metal bar becomes electrically isolated so that no current flows. To prevent these conditions, the metal bars had to be removed much before complete dissolution.